MQTT-SN Feature: Allow MQTT to function across connectionless transports (like UDP)

To work on connectionless, unreliable network transports (UDP is the most important, perhaps canonical example), packet retries are essential.

Some transports may not guarantee the integrity of packets, but I don't think we should introduce error correction into MQTT due to the complexity. If you want to implement MQTT over such transports, then you would have to add error correction yourself (outside of MQTT).

I propose we should use the characteristics of UDP as the basis for requirements on the underlying connectionless transports.